Automated voice processing systems have become widely prevalent over the past several years. In such systems, a remotely located user accesses the system by dialing a predetermined telephone number, thereby effectuating a connection from the remote telephone terminal to the voice processing system. The voice processing system, typically a personal computer based device, receives commands from the remote user in the form of dialed digits. The voice processing system then acts upon the dialed digits by performing various functions such as reading account balances, issuing further instructions, etc.
In many present day systems, such digits are dialed by the user entering Dual Tone Multi Frequency (DTMF) digits. In accordance with the DTMF system, a predetermined set of two frequencies is assigned to each digit. The particular digit entered by a remote user is decoded by the voice processing system by analyzing the signal received in order to determine which two particular frequencies are contained therein. The two frequencies identify the particular digit dialed.
One problem with these systems is that in many areas of the United States and abroad, the majority of the telephone switches are not equipped with DTMF capability. Rather, many phones utilize Dial Pulse Digits (DPD), the more conventional type of telephone in which digits are entered by rotating and then releasing the dialer, thereby allowing it to rotate back to its original position. As the dialer rotates back to its original position, a plurality of "clicks" are produced, whereby the loop current to the central office is repetitively connected and disconnected. This sequence of clicks is termed "the DPD sequence," and is an analog signal which is, in the ideal situation, substantially periodic. The DPD sequence comprises a plurality of pulses, where each pulse. is one period of a substantially periodic signal. Each pulse may include one or more clicks
The number of clicks N produced is related to the dialed digit by a simple arithmetic formula (e.g. 2N+1, 2N, etc.) which varies from system to system. Additionally, there is sometimes an extra click or two at the beginning of the sequence, or the end. In short, many properties of DPD sequences vary from system to system, which properties include (i) number of clicks per pulse (ii) presence or absence of a pulse at the start of the sequence, and (iii) presence or absence of a pulse at the end of the sequence.
In such systems, the voice processing system must decode the particular digit dialed by counting the number of pulses, or the length of the sequence of clicks, or by measuring some other parameter of the sequence of pulses in order to determine the particular digit dialed. One common technique is to ascertain the period of the DPD signal, and divide the length of the signal by the period thereof.
The problem with such systems is that they are not nearly as accurate as DTMF systems. Several characteristics of DPD systems cause such systems to have a much higher error rate than their DTMF counterparts. One major problem is that the DPD sequence is not perfectly periodic. Rather, the duration of each pulse, and the time periods therebetween, may vary. On some DPD telephones, the variance is so great that the sequence can not even be considered periodic. Thus, it becomes difficult to detect the digit by simply dividing the length of the signal by the period thereof. Accordingly, it is extremely difficult to configure a voice processing system to accurately detect DPD digits dialed from different telephones which produce greatly varying signals.
In an attempt to solve the above problem, the technique of a training digit has been utilized by DPD voice processing system providers. The training digit systems prompt the user to enter a training digit when the call is first connected. The system may play an audible message, for example, which says "enter the number 0." When the user dials the zero, a voice processing system can then measure the length of the sequence produced and adjust its detection criteria to optimize detection of digits having the parameters contained in the DPD sequence which produces the training digit. For example, future dialed digits may be detected by comparing the sequence thereof to the sequence of the training digit. If the training digit is a zero, then a digit having a sequence four-tenths the length of the training digit is a four.
A problem with the use of a training digit is that it is annoying for an end user. Before entering data, the user must dial the training digit, which wastes time. However, without the training digit, the "bad" rotary telephones which produce DPD sequences that are not periodic will cause errors at the voice processing system.